The present invention relates to steam reformers. More particularly, the present invention relates to steam reformers for catalytically converting a fuel into a reformate stream comprising hydrogen. The present steam reformer incorporates multiple reformer tubes and a multiple element burner.
A catalytic hydrocarbon fuel steam reformer converts a fuel stream, comprising, for example, desulfurized natural gas, light distillates, methanol, propane, naphtha, kerosene, and/or combinations thereof, and water vapor into a hydrogen-rich reformate stream. The hydrogen-rich reformate stream is generally suitable for use as a fuel gas stream directed to the anode of a fuel cell after passing through a water gas shift reactor and other purification means such as a carbon monoxide selective oxidizer or a pressure swing absorption (xe2x80x9cPSAxe2x80x9d) unit. In the conversion process, the raw hydrocarbon fuel stream is typically flowed through a catalyst bed or beds contained within reactor tubes mounted in a reformer vessel. The catalytic conversion process is normally carried out at elevated catalyst temperatures in the range of about 600xc2x0 C. to about 800xc2x0 C. Such elevated temperatures are typically generated by the heat of combustion from a burner incorporated into the reformer.
The search for alternative power sources has focused attention on the use of fuel cells to generate electrical power. Unlike conventional fossil fuel power sources, fuel cells are capable of generating electrical power from a fuel stream and an oxidant stream without producing substantial amounts of undesirable byproducts, such as sulfur oxides, nitrogen oxides or carbon monoxide. However, the commercial viability of fuel cell systems depends in part on the ability to efficiently and cleanly convert conventional hydrocarbon fuel sources, such as natural gas (methane) or methanol, for example, to a hydrogen-rich reformate gas stream. Properly designed catalytic steam reformers can generate the required reformate gas streams with increased reliability and decreased cost.
As to reliability and cost, conventional industrial catalytic steam reformers have at least two major disadvantages with respect to fuel cell use. First, because conventional industrial reformers operate at very high temperatures and pressure differentials, the reformer tubes that contain the catalyst must be constructed of rugged, thick walled portions of expensive materials capable of withstanding high-temperature operating conditions. Additionally, conventional industrial steam reformers also tend to be quite large, which again impacts material costs.
Smaller steam reformers have also been designed for use in fuel cell system applications. Such steam reformers have employed single-tube and multiple-tube designs. The smaller steam reformer designs have at least two major disadvantages in fuel cell system applications.
First, current steam reformer designs tend to lack quick start-up capability, with start-up times typically of from about one to four hours. Lack of quick start-up capability can be problematic in some fuel cell applications, particularly where the reformer is expected to have a relatively short duty cycle.
Some current steam reformer designs utilize a multi-element burner, but these burners do not adequately provide for quick start-up and/or lack the flexibility to efficiently operate on multiple fuels, including for example, natural gas, fuel cell anode exhaust or PSA off-gas. For example, in a fuel cell power plant a steam reformer may be used to convert natural gas into a hydrogen-rich fuel stream, and it is desirable to have a burner capable of operating on natural gas and air (start-up mode), a reformate stream and air (transition or xe2x80x9chot standbyxe2x80x9d mode), and the fuel cell anode and cathode exhaust streams (normal operation mode).
Second, as part of fuel processing systems in fuel cell-related applications or merchant hydrogen production, for example, current steam reformer designs are less than cost-effective. For example, high-pressure burners and/or reformer vessels increase the parasitic load on the fuel processing system due to associated compressors, thereby decreasing efficiency and increasing cost and complexity. Conversely, in merchant hydrogen production applications, a low-pressure reformer vessel increases the fuel processing system parasitic load because of the associated process gas or syngas compressor that is required. In addition, current steam reformer designs tend to be relatively complex, resulting in increased manufacturing costs and reliability concerns.
It is desirable for a steam reformer to be able to start up relatively quickly, and to be able to operate efficiently without adding undue complexity or cost. At the same time, it is desirable for a steam reformer to be low-cost, scalable, and compatible with a variety of fuel processing systems.
A compact, multiple tube steam reformer converts a fuel into a reformate stream comprising hydrogen. In one embodiment, the present steam reformer comprises a closed vessel and a burner disposed within the vessel. The burner comprises:
(a) a burner fuel manifold for receiving and distributing a burner fuel stream, the burner fuel manifold comprising a plurality of burner fuel distribution tubes, each of the burner fuel distribution tubes having an inlet end and an outlet end, the burner fuel distribution tubes disposed in a separator member;
(b) an oxidant manifold for receiving and distributing an oxidant stream, the oxidant manifold comprising a plurality of oxidant distribution tubes, each of the oxidant distribution tubes having an inlet end and an outlet end, the oxidant distribution tubes extending through the burner fuel manifold and fluidly isolated therefrom; and
(c) a start fuel manifold for receiving and distributing a start fuel stream, the start fuel manifold comprising a plurality of start fuel distribution tubes, each of the start fuel distribution tubes having an inlet end and an outlet end, the start fuel distribution tubes extending through the oxidant manifold and fluidly isolated therefrom.
The outlet end of each of the oxidant distribution tubes extends into the inlet end of one of the burner fuel distribution tubes, forming a first gap between the outer wall of the oxidant distribution tube and the inner wall of the burner fuel distribution tube, and the outlet end of each of the start fuel distribution tubes extends into the inlet end of a corresponding one of the oxidant distribution tubes, forming a second gap between the outer wall of the start fuel distribution tube and the inner wall of the oxidant distribution tube.
In another embodiment, the reformer comprises a closed vessel and a burner disposed within the vessel. The burner comprises:
(a) a start fuel manifold for receiving and distributing a start fuel stream;
(b) an oxidant manifold for receiving and distributing an oxidant stream, the oxidant manifold comprising a plurality of oxidant distribution tubes, each of the oxidant distribution tubes having an inlet end and an outlet end, the oxidant distribution tubes disposed in a separator member; and
(c) a burner fuel manifold for receiving and distributing a burner fuel stream, the burner fuel manifold comprising a plurality of burner fuel distribution tubes, each of the burner fuel distribution tubes having an inlet end and an outlet end, the burner fuel distribution tubes extending through the start fuel manifold and the oxidant manifold and fluidly isolated therefrom.
The outlet end of each of the burner fuel distribution tubes extends into the inlet end of a corresponding oxidant distribution tube, forming a gap between the outer wall of the burner fuel distribution tube and the inner wall of the oxidant distribution tube, and wherein the start fuel manifold has one or more openings therein associated with at least a portion of the burner fuel distribution tubes.
The burner fuel distribution tubes and oxidant distribution tubes may be arranged in a hexagonal array. The gaps formed between the burner fuel distribution tubes and the oxidant distribution tubes may be annular gaps.
The separator member of the present steam reformer may comprise insulating material. The insulating material may comprise a ceramic, for example.
In the latter embodiment of the present steam reformer, the openings in the start fuel manifold may comprise one or more discrete openings distributed around the circumference of at least a portion of the burner fuel distribution tubes. The openings may be asymmetrically distributed around the circumference of each of the burner fuel distribution tubes.
Alternatively, at least a portion of the burner fuel distribution tubes may extend through a corresponding opening in the start fuel manifold, forming at least one gap between the outer wall of each of the burner fuel distribution tubes and the corresponding opening. The gaps between the outer walls of the burner fuel distribution tubes and the corresponding openings may be annular gaps. Alternatively, the gap between at least a portion of the burner fuel distribution tubes and corresponding openings of the start fuel manifold may comprise one or more gaps distributed around the circumference of at least a portion of the burner fuel distribution tubes. In this case, the gaps may be asymmetrically distributed around the circumference of each of the burner fuel distribution tubes.
The present steam reformer may further comprise: a plenum disposed within the vessel for receiving reformate, the plenum having a floor plate; a bottom pan disposed within the plenum, the bottom pan substantially in the shape of an inverted bowl and having at least one hole therein; and, a reformate outlet extending from the floor plate into the plenum in the region defined by the cooperating surfaces of the bottom pan and the floor plate.
In another embodiment, the present steam reformer comprises:
(a) a closed vessel;
(b) a plenum disposed within the vessel for receiving reformate, the plenum having a floor plate;
(c) a bottom pan disposed within the plenum, the bottom pan substantially in the shape of an inverted bowl and having at least one hole therein; and
(d) a reformate outlet extending from the floor plate into the plenum in the region defined by the cooperating surfaces of the bottom pan and the floor plate.
The present steam reformer may further comprise a plurality of substantially cylindrical reformer tubes disposed within the vessel, each of the reformer tubes comprising a concentric inner gas return tube and an outer annular catalyst bed defined by the volume between the reformer tube and the inner gas return tube. The reformer tubes may further comprise at least one stabilizer member connected to the inner gas return tube and interposed between the reformer tube and the inner gas return tube. The stabilizer member may comprise at least one annular perforated plate extending radially from the inner gas return tube, or at least one fin extending axially from the inner gas return tube, for example.
The present reformer may further comprise a vaporizer disposed within the vessel for receiving and vaporizing a stream comprising water. The vaporizer may comprise a finned tube helical coil, or a corrugated tube helical coil, for example.